$-9df - 2e + f - 1 = 7e - f - 5$ Solve for $d$.
Explanation: Combine constant terms on the right. $-9df - 2e + f - {1} = 7e - f - {5}$ $-9df - 2e + f = 7e - f - {4}$ Combine $f$ terms on the right. $-9df - 2e + {f} = 7e - {f} - 4$ $-9df - 2e = 7e - {2f} - 4$ Combine $e$ terms on the right. $-9df - {2e} = {7e} - 2f - 4$ $-9df = {9e} - 2f - 4$ Isolate $d$ $-{9}d{f} = 9e - 2f - 4$ $d = \dfrac{ 9e - 2f - 4 }{ -{9f} }$ Swap the signs so the denominator isn't negative. $d = \dfrac{ -{9}e + {2}f + {4} }{ {9f} }$